Some populations have a higher prevalence of specific cancer-associated alleles than others. This may result from a founder effect, which occurs when a population undergoes rapid shrinkage and then expansion in an isolated setting. In a population that is geographically or reproductively isolated, an individual called a founder carries or develops a germline mutation that is rare in the general population.
![Prevalence and Founder Effect](https://webarchive.library.unt.edu/eot2008/20081006124647im_/http://www.cancer.gov/images/Documents/a3a0ee76-1bc5-4490-aeaa-2248702c668f/cancer37.jpg)
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